1 **U NITED STA** 

**TES ENVIRONMENTA L PROTECTION A**  **GENCY** NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWO OD ROAD ANN ARBOR, MI 48105

May 4 , 20 12 OFFICE OF AIR AND RADIATION MEMORANDUM
SUBJECT: NO X Emissions from DPF Regeneration
FROM: Lauren Steele, Office of Transportation and Air Quality
TO: Docket EPA

- HQ

- OAR

- 2011

- 1032



### Estimate **s of Emissions**  **of Oxides of Nitrogen**  **from Regulated Emergency Vehicles**  The Heav y - Dut y Highway Rule was projected to reduce oxides of nitrogen (NO X ) exhaust emissions from the heav y - dut y on - road fleet b y 1.76 million tons in 2020, and by 2.48 million tons i n 2030. In that rule’s final Regulatory Impact Analy sis, EPA projected that in 2010 the controlled heav y - dut y fleet would emit approximately 2 million tons of NO X emissions, decreasing to about 600,000 tons in 2020 and about 300,000 tons in 2030, as a res ult of that rule. 1 For purposes of this memo, we are considering that rule’s control case as the current fleet baseline scenario. As noted in Section VII.B of the proposed rule preamble, class 3 through 8 fire apparatus and ambulances represent about 0.8 percent of annual new heav y - duty truck registrations. 2 If we assume these vehicles have the same emissions profile over time as other vehicles, we could estimate that their annual emissions collectively may also represent a comparable fraction of the flee t emissions in a given ye ar. For the y ears projected in the RIA for the Heav y - Dut y Highway Rule, this would indicate emergency vehicles may

have emitted on the order of 16 ,000 tons of NO X in 2010, may emit about 5 ,000 tons in 2020, and may emit about 2,00 0 tons in 2030.

1 Final Regulatory Impact Analysis for the “ 2007 Heavy

- Duty Highway Rule ,” EPA420

- R

- 00

- 026, December 2000 , Chapter II Health and Welfare Concerns and Emissions Benefits, Table II.B

- 4, available at http://www.epa.gov/otaq/highway

- diesel/regs/ria

- ii.pdf 2 ICCT, May 2009, “Heavy

- Duty Vehicle Market Analysi s: Vehicle Characteristics & Fuel Use , Manufacturer Market Shares .”2 However, we have information that these vehicles have longer than average useful lives, leading to a slower fleet turnover rate. We also have information that these vehicles have below average vehicle miles traveled, with a large fraction of their engine hours at low load or idle. While these two factors would affect the emissions inventory in opposite directions, they are unlikel y to cancel each other out. Because we do not have enough data to quantify

how these factors affect the emission estimates, we will use the values presented above for purposes of the current analy sis. On a per

- truck basis, we can estimate w hat the NO X exhaust emissions from a certif ied engine would be if it emitted NO X at the level of the standard, 0.2 gram per brake

- horsepower

- hour (g/bhp

- hr) . As an illustration, we have estimated what that might represent in terms of emissions from a truck operating at 200 horsepower over a t ypical ye ar with 1,200 operating hours. This would be representative of a 500

- hp pumper truck with an average engine operating load of 40 percent of capacity . This illustration would yi eld a h ypothetical fire truck that would emit about 100 pounds of NO X exhaus t emissions in a y ear, or 0. 05 ton per year
. These estimates are provided as a basis for comparison, as the following sections explore some different scenarios. 

### Estimates of NO **X** 

### Em **ission s**  **from Regeneration** The estimates of NO X emissions for certified engi nes generally include emissions during periods when the diesel particulate filter (DPF) is regenerating. During both active automatic and parked manual regenerations, emission rates may increase for some pollutants, especially

NO X when NO X - reducing aftertreatment devices are not present downstream of the DPF . The higher than normal temperatures during active regeneration with high rates of oxidation occurring across the cataly st create conditions conducive to NO X formation. Because of the potential effect on emissions, engine manufacturers are allowed to provide adjustment factors that account for the excess emissions during DPF regeneration, as part of the certification process. During a series of certification tests, the High NO X Emission Factor (EFHNO X ) represents the NO X emission rate during an active regeneration, and the DPF regeneration frequency represents the fraction of tests during which an automatic active regeneration occurs. Passive regenerations would not be expected to increase NO X emissions, thus they are not counted in the DPF regeneration frequency for testing purposes . Figure 1 illustrates the relationship between the EFHNO X and DPF regeneration frequency , based on EPA certification data for MY 2008 engines over the transient test cy cle.3 0 1 2 3 4 5 6

0% 2% 4% 6% 8% 10%**H i** **g h** **N O** **X E** **m i** **s s** **i o** **n R** **a t** **e (** **g /** **b h** **p -** **h r** **)**

**DPF Re ge**  **nera ti**  **on Freque ncy**  

# 2008 T **ra nsient** **E FHNOx**  **wit h**  **Fr eq**  **DP F** 

**Figure 1**  **: High NO**  **X Em**  **ission Factor and DPF Regeneration Frequency**  As noted above, excess emissions from DPF regenerations are generall y accounted for as part of the engine certification process. Thus, during DPF soot accumulation mode the NO X emissions may be lower than the standard for some engines, and we see a general inverse relationship between DPF regeneration frequency and the EFHNO X . We can also observe from Figure 1 that the NO X emission rate during a regeneration may

be many times higher than the standard. The amount of excess NO X emissions due to DPF regeneration would generally be expected to be small on a vehicle whose engine is equipped with downstream NO X emissions control, such as select ive cataly tic reduction. EPA’s certification data for MY 2011 engines indicate th e EFHNO X are generall y below 1 .0 g/bhp

- hr, with some outliers. DPF regeneration frequencies for MY 2011 engines do not differ substantially from those of MY 2008 engines. For more information on DPF regeneration frequencies, see the docket memo entitled Fuel Use with Dosing for DPF Regeneration, submitted to the docket for this rulemaking. 

### Estimates of Excess NO **X** 

### Em **issions from Emergency Vehicles w**  **ith More Frequent Regeneration**  **s and Higher Emission Rates**  As noted in the memorandum to the docket on fuel dosing, it is possible that some manufacturers will submit applications for AECD’s or field modifications for engines on emergency

vehicles with liberalized parameters under which automatic active and/or parked manual regenerations may occur, potentially

increasing the NO X emissions from those vehicles . It is also possible that manufacturers may submit requests for engine recalibrations that enable more frequent passive regeneration and rel y less on active reg eneration, potentially

changing the NO X emissions in more complex way s
. It is thus difficult to predict the effects this rule may have on the NO X emissions from emergency vehicles.4 As described in the preamble to the proposed rule, EPA has proposed that engines with approved emergency vehicle AECD’s would not need to include these excess emissions during certification. In other words, the engines would be tested with emergency

vehicle AECD settings inactive. For purposes of this illustration, we assume the regeneration NO X in the base case is included in the certified emission level, and excess regeneration NO X due to the AECD would be above the certified emission level. Without speculating what fraction of the emer gency vehicle fleet may

receive AECD’s or field modifications with NO X recalibrations, for this illustration we will focus on a per - truck emissions estimate. For this h ypothetical truck, w e are assuming that the current certified configuration (the base c ase) includes a DPF regeneration frequency (FREQ DPF ) of 5 percent, a high NO X emission rate of 2.5 g/bhp/hr (EFHNO X during regeneration), and a low NO X emission rate of 0.2 g/bhp

- hr ( EF L NO X when not regeneratin g ). This example is purely hy pothetical

– EPA is not aware of an engine with these specific emissions characteristics. The alternate cases explore the emissions effects when the DPF regeneration frequency (FREQ DPF ) increases to 20 percent, and when the EFHNO X increases to 5.0 g/bhp

- hr, while holding other parameters the same. The values presented in Table 1 have been calculated using the following formula for the average NO X emission rate (EF A ):

EF A = (FREQ DPF )(EF HNO X ) + (1

- FREQ DPF )( EF LNO X )
Where FREQ DPF = Base 5% or Alternate 20% EF HNO X = Base 2.5 g/bhp

- hr or Alternate 5.0 g/bhp

- hr EF LNO X = 0.2 g/bhp

- hr in all cases To extrapolate the average emission rate (EF A ) to an annual value , w e are continuing with the example of a truck with a 500

- hp engine operating at an average load of 40%, for a work ing power of 200 hp and average of 1,2 00 e ngine operating hours in a ye ar
. **Table 1**  **: Per**
**- Truck**  **NO X**  **Emission Esti**  **ma tes by Regeneration Percent**  Scenario EF A Pounds per Year Tons Per

Year Base Case 0.32 170 0.08 Base FREQ DPF In creased EFHNO X to 5g/bhp

- hr 0.44 230 0.12 In creased FREQ DPF to 20 Percent Base EFHNO X 0.66 350 0.17 In creased FREQ DPF to 20 Percent In creased EFHNO X to 5g/bhp

- hr 1.16 610 0.315 As shown in the table, if the both regeneration frequency and the high NO X emission rate were to increase as described, the annual NO X emissions from this truck could increase fourfold. We believe this is a highly conservative estimate. As shown in Figure 1, these parameters have an inverse relationship, so it is highly unlikel y that both parameters would be maximized in the same engine configuration. W e present below in Table 2 the emissions estimates using an alternate calculation method that is based on the regeneration interval. Similar to the method used in the docket memo entitled PM Emissions I mpacts, we have estimated in the base case that a single regeneration event has a duration of 30 minutes, and the interval between regenerations is 25 engine hours. In the alternate cases we explore the emissions effects if the rege neration interval is reduced to 12 or 8 engine hours, and each case uses the same EF HNO X and EF LNO X as in the previous illustration.

**Table 2**  **: Per**
**- Truck NO**  **X Emission**  **Esti ma**  **tes by Regeneration Interval**  Regeneration Interval Operating Time

Regenerating EFHNO X

(g/bhp

- hr) NO X Emissions

(TPY) 25 hours 2.0% 2.5 0.06 25 hours 2.0% 5.0 0.08 12 hours 4.0 % 2.5 0.08 12 hours 4.0 % 5.0 0.10 8 hours 5.9 % 2.5 0.09 8 hours 5.9 % 5.0 0.13 The values in Table 2 indicate that if regenerations were to occur at 8 engine

- hour intervals instead of 25 engine

- hour intervals, with the EFHNO X emission rate doubling from 2.5 to 5 g/bhp

- hr, then the annual NO X emissions from this hy pothetical fire truc k could double. All of the above examples assume that no additional NO X control occurs downstream of the DPF. However, the majorit y of the heav y - duty fleet of MY 2010 and later is equipped with selective cataly tic reduction NO X aftertreatment sy stems. When active, this technology reduces NO X emissions dramatically , including an y excess NO X emissions due to D PF regeneration.